Improving power consumption in NFC and HF RFID card detection systems

Battery-powered Near Field Communication (NFC) and radio frequency identification (RFID) reader applications must have a defined and limited energy consumption budget and be low cost for viable product realization. Techniques and strategies for card presence detection have emerged over the years that attempt to address both concerns. This article will contribute to the previously identified card detection techniques and strategies by presenting an alternative solution including the addition of a simple circuit and a small firmware control logic loop to an existing design, which offers a dramatic improvement in current savings and results in longer battery life.

Additionally, a brief overview of the benefits and disadvantages of the most common implementations of battery-powered RFID and NFC readers will be given, and in-depth details provided on the innovative technique and approach, developed by leveraging both ultra-low power 16-bit MCUs and highly integrated NFC/RFID reader/writer integrated circuits (ICs) in battery-powered NFC and HF RFID reader applications or designs.

Criteria for system design decisions Designers and developers of low-power NFC or RFID systems usually have a list of key requirements that are driven directly by their target markets or focused end equipment. These requirements might include access control (building access), digital door locks, smart utility meters (for prepayment, technician access, or firmware upgrades), portable speakers, handheld inventory control, handheld data logging collection or medical diagnostic equipment, and mobile/handheld ticketing or payment terminals. All of the end equipment examples have similar key "care-abouts" such as:

Total system cost: Cost must be optimized.

Electrical design: Platform or modular designs are becoming more prevalent, with emphasis on being deployable worldwide. For example, a design that can be used in any country like a 13.56MHz NFC/RFID system.

Mechanical design: It must be robust, safe, and provide various levels of protection against vandalism.

User friendly and intuitive: Users should never need much training to interact with them, so the design must always lend itself to an easy user experience.

Low power consumption: A power budget is more important than ever, and when implemented as described later in this article, can be a major differentiator and add competitive advantage to designs.

Previous approaches/implementationsAn overview of other approaches to card detection in battery-powered NFC and RFID reader applications are presented for review in order to provide the reader with a basic understanding of what has been done in the past, and then be able to fully appreciate the advanced technique.

Mechanical or optical: This approach comprises a card slot equipped with a mechanical switch or break beam detector that triggers a read cycle of the NFC/RFID card when activated by the card. This implementation is a carry-over from magnetic card swipe applications (Figure 1).

Figure 1. Examples of mechanical and optical card detection systems.

The advantages of this approach include very low power consumption; and the fact that many consumers are familiar with this interaction. Unfortunately, this implementation is challenged by limited card form factors and a lack of robustness that allows the system to be easily broken or rendered unusable.

Resonator implementation: This method includes an NFC/RFID reader equipped with an additional resonator, oscillator, or crystal and resonant coil. The MCU enables the signal to be generated as a short burst (20 to 50µs) and detects the antenna dampening (Figure 2).

Figure 2. Diagram of a typical resonator implementation, which includes an additional resonator, oscillator, or crystal and resonant coil.(Click Here to see a larger, more detailed version of this image)

This is commonly seen in battery-powered NFC/RFID access control/digital door lock applications. Low power consumption and the need for no additional mechanical components make this a compelling implementation, but these types of systems are challenged by limited detection distance and additional electrical components required, including two antennas (RFID and card sniffer) or switch (capable of handling 13.56MHz signal at power levels up to +23dBm, a second oscillator or crystal with a fast run-in time, and also EMC regulation compliance for frequency and accuracy.

Capacitive proximity sensor approach: This implementation has an NFC/RFID reader antenna that also comprises areas used for a capacitive sensor. Capacitive proximity sensors detect "target" objects due to their ability to be electrically charged. Since non-conductors can even hold charges, just about any object can be detected with this type of sensor and has made this approach popular in mobile/handheld applications. The reader sends out activation commands for the protocols and indicates the types of cards expected each time the sensor detects any change (Figure 3).

Figure 3. Diagram of a capacitive proximity sensor approach to card detection systems.(Click Here to see a larger, more detailed version of this image)

The advantages of this approach include a flexible firmware solution, good detection range and flexibility to be used with all NFC/RFID card types. On the other hand, high power consumption, occasional false triggers and wake-ups due to the detector measuring the electric field and NFC/RFID card using the magnetic field, and the need for many additional components to implement this type of system are all major disadvantages to this approach.

Received Signal Strength Indicator (RSSI) implementation: This approach employs dual receivers, each with an RSSI detector, inside the IC. TI's TRF79xxA family of readers provides direct support for this approach. The reader system is normally in an RSSI detection loop, issues activation commands (by protocol), and then reads out the latched RSSI value (in the RSSI register).

Figure 4. Diagram of the Received Signal Strengths Indicator (RSSI) implementation.(Click Here to see a larger, more detailed version of this image)

Based on the RSSI value returned, the firmware logic would then issue either additional commands (for that protocol) or remain in RSSI detection loop. This implementation does not require any additional components, is a flexible firmware solution, can achieve good detection range, and can be used with all NFC/RFID card types. Alternatively, this system has higher power consumption and peak current, and has medium detection resolution, both of which are only as compared to other solutions.

A new solution for ultra-low-power (ULP) card presence detectionAn alternative and improved solution combines some of the older ideas presented above with new approaches. The main differences include carefully chosen values of the analog circuit components (see items in blue, Figure 5), and the 16-bit MCU firmware that leverages the low-power modes and port settings available in the MCU itself, and the logical loop control of the ULP NFC/RFID IC power modes. Compared to the implementations listed above, the technique outlined in this section dramatically lowers power consumption.

The evaluations in this approach use a TRF79xxA evaluation module (EVM) from Texas Instruments. The complete system is shown in Figure 5. The features in blue indicate which additional components are needed for the card proximity detector circuit.

Figure 5. Diagram of the improved solution for ultra-low power card presence detection proposed in this article.(Click Here to see a larger, more detailed version of this image)

The basic idea behind this improvement is to combine part of the resonator concept with the existing NFC/RFID reader system. The existing NFC/RFID reader system in this example uses the EVM mentioned above, which includes the TRF7970A transceiver and the ultra-low power MSP430F2370 or MSP430G2xx MCUs that have an MSP430 Comparator A+ module integrated on-board and the ability to accept a high frequency (HF) clock in (for synchronization purposes).

This card presence detection system senses the card by measuring the decay time of a transmitter signal after it has been turned off. When a card is in the field of the transmitter, power transfer occurs and with more current, the voltage on the transmitter output increases. The closer the card is to the reader the higher the voltage will be from the 'not present' state. Measuring the decay of this signal by a comparator creates a simple analog-to-digital converter (A/D), since the voltage is effectively what is being measured. Longer decay times, until the output signal crosses a lower threshold, indicate higher voltages and shorter decay times indicate lower voltages.

The power savings are quite substantial because of the always-running transmitter. In a system where there are three polls per second, the system is only active approximately 1 percent of the time. In a sleep state, the NFC/RFID IC consumes almost no power, while the MCU is consuming a negligible amount of current (approximately 0.8µA). In the active state, which lasts several milliseconds, the NFC/RFID IC transmitter is quickly turned on, initialized, and a continuous wave (CW) transmitter burst is performed for approximately 20µs. Before it is turned off, the comparator is initialized and a timer is started to measure the time. The timer runs until the comparator issues an interrupt, indicating that the threshold voltage has been crossed. The timer time at this point is the decay time of the signal. As mentioned earlier, longer times indicate power coupling, which means that a card may have been in the field.

To determine if a certain time measurement indicates that a card is in the field, it is run through what is referred as "automatic calibration algorithm". To explain this need, let's examine what would happen without it. In its absence, a specific time value, that once exceeded, would indicate that a card is in the field. However, due to power supply or temperature drifts, this threshold time would be naturally crossed periodically, causing false positives. False positives are very undesirable in this system as they cause a reading process to happen, which uses a lot of power.

In this ULP card presence detection implementation, the use of an integrated comparator module is a critical detail. Its integration into the MCU guarantees a reduced Bill of Materials (BOM) count compared to the resonator approach previously presented. Additionally, the mixed signal MCU approach (integrated analog + clever firmware) allows for a wide range of customization and optimization.

Signal analysisA graphical representation of one cycle of the card presence detection process is shown in Figure 6, where the RF Carrier (RF1) is turned on for a short period. During the transmit on time, the RC network used in the circuit is charged up quickly, then when the transmitter is turned OFF, the same capacitor in that RC network begins to discharge and the input to the comparator (DEMOD) signal is used to make the measurement.

Figure 6. Graphic showing one cycle of the card presence detection process, where the RF Carrier (RF1) is turned on for a short period.(Click Here to see a larger, more detailed version of this image)

Once the transmitter is turned off, the timer inside the MCU starts counting until the CAOUT signal is asserted. The count between the transmitter turning off and CAOUT being asserted is the sample time. The NFC/RFID reader was operated at +3VDC, representing the common voltage level used in battery-powered applications today (for example, one CR2032, or two 'AAA', or two 'AA', or two 'C', or two 'D' batteries in series).

While the previous illustration was theoretical, below is an actual measurement cycle, using an oscilloscope. The transmitter (shown green on channel 3) is turned on for ~20µSec and the sense line (shown red on channel 2) goes high as it monitors the transmitter output. The transmitter is then turned off and the sense line starts to decay as the capacitor is being discharged. Eventually the comparator's voltage threshold is crossed (at approximately 1.5V) and the comparator output (CAOUT, shown in purple on channel 4) is triggered. This interrupts the MCU and the time from the transmitter turning off to the CAOUT going high is taken, which becomes the sample time. In this case, no card is in the field.

Figure 7. Graph showing no card detected in the field.(Click Here to see a larger, more detailed version of this image)

In Figure 8, the view on the oscilloscope is zoomed out (as compared to the figure above) and a card has been detected in the field, which triggers a card read cycle, (see ISO read cycle annotation).

Figure 8. Graph showing a card being detected in the field triggering a card read cycle.(Click Here to see a larger, more detailed version of this image)

Meanwhile, Figure 9 shows a breakdown of current consumption during each stage or step of the card presence detection monitor process run three times in one second. This is where the three times a second interval (approximately 336mSec) is coming from, and if it needs to be sped up or slowed down, this is a simple change to make. Also, in the case of building access systems that are "learning" an occupant's behaviors and adjusting energy consumption accordingly, this would be the variable that the firmware algorithm would be changing.

Figure 9. A breakdown of current consumption during each stage of the card presence detection monitor process.(Click Here to see a larger, more detailed version of this image)

The first spike of the current graph is where the bypass capacitors are being charged instantaneously. Immediately afterwards is the initialization of the transceiver and then the sleep period. The last spike is for the actual RF carrier transmitter burst and the sampling period afterwards.

(1) Assumption: VCC = 3.0V, polling: three times per second(2) 304µs is the time until de-assertion of EN signal. 488µSec is the time until current from active cycle returns to normal (using 304µSec).

The table below shows the average current that can be measured, based on various reasonable polling cycles per second. The example described in this article is using a polling cycle of 3Hz (or three times per second).

SummaryIt is critical to realize that average current consumption decreases as the wait times in between the detection cycles increase. Most solutions that might claim to have low-power card detection built in are most likely putting out a voltage on their antenna coil (like in this example), but then are using a DAC to measure current changes. The proposed alternative implementation puts out a voltage and measures a change in that voltage directly using the comparator and timer hardware that is already built in to the mixed signal MCU.

Using this approach, we can achieve lower power consumption by comparison and only improve as time in between "sniff" cycles increases. This system solution can be used in battery-powered applications with very little extra cost. Along with an automatic calibrating algorithm, voltage or temperature drifts do not cause unnecessary false positives and thus conserve power.

Developers of battery-powered NFC/RFID systems that need to conserve as much energy as possible as part of their value proposition need to consider the superior card presence detection solution outlined in this article as the path to achieving the ultra-low power card detection operations in their application.

About the authorsJosh Wyatt is Applications Team Manager in the Safety and Security NFC/RFID team at Texas Instruments Incorporated (TI). Alexander Kozitsky is an Applications Engineer in the Safety and Security NFC/RFID team Texas Instruments Incorporated (TI).

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